The present invention relates to a method for preventing the corrosion of transition metal-based cathodes used in electrolytic diaphragm-type cells. Such cathodes are advantageous due to their ease and low cost of fabrication and their low overvoltage characteristics when used, for example, in the electrolysis of aqueous solutions of alkali metal halides to produce alkali metal hydroxides and halogens.
A typical electrolytic alkali metal halide cell is an enclosed container which is physically partitioned into at least two distinct regions or chambers by means of a permeable intermediate barrier or cell separator, such as an asbestos diaphragm or synthetic microporour separator. During the electrolysis of an alkali metal halide solution, hydrogen and alkali metal hydroxide are formed at the cathode while chlorine and oxygen are formed at the anode. When the alkali metal is sodium, the electrolytic solution in the cathode compartment, i.e. the catholyte, may contain approximately 12%-17% NaOH, 15%-20% NaCl, and negligible, e.g. about 10 p.p.m., NaOCl. Under conditions of normal operation, sodium hypochlorite will generally not cause deterioration of the cathode coating material. However, when the current flow to the cell is interrupted, such as when the cell is subjected to routine maintenance during a shutdown period, the concentration of sodium hypochlorite in the catholyte increases significantly to amounts of up to 2 gms./liter or more. Such concentrations of sodium hypochlorite can have an immediate adverse effect on the transition metals contained in the cathode, causing such metals to dissolve in the solution and ultimately leading to the failure of the cathode coating. "Transition metal", in the context of the present invention, includes iron, cobalt, nickel, their oxides and combinations of alloys thereof.
The rapid increase in sodium hypochlorite concentration in the catholyte during periods of current interruption is caused by the convective flow and diffusion of hypochlorite ions from the anolyte. At the cathode, the sodium hypochlorite is reduced to sodium chloride and water as follows: EQU NaOCl+2H.sup.+ +2e.sup.- .fwdarw.NaCl+H.sub.2 O (1)
The corresponding oxidation of the cathode transition metal, illustrated below by nickel, can be designated as follows: EQU Ni.fwdarw.Ni.sup.++ +2e.sup.- ( 2)
The overall reaction can thus be designated as: EQU Ni+NaOCl+2H.sup.+ .fwdarw.Ni.sup.++ +NaCl+H.sub.2 O (3)
The nickel in the cathode is thus ionized and becomes soluble in the catholyte causing dissolution of the coating.
In recent years, increasing attention has been directed toward improving the hydrogen overvoltage characteristics of electrolytic cell cathodes. In addition to having a reduced hydrogen overvoltage, a cathode should also be constructed from materials that are inexpensive, easy to fabricate, mechanically strong, and capable of withstanding the environmental conditions of the electrolytic cell. Iron or steel fulfills many of these requirements, and has been the traditional material used commercially for cathode fabrication in the chlor-alkali industry. However, since steel cathodes generally exhibit overvoltage in the range of from about 300 to 500 millivolts under typical cell operating conditions, i.e. at a temperature of about 90.degree. C. and a current density of from 100 to 200 milliamperes per square centimeter, efforts have focused on improved cathode coatings having significantly reduced hydrogen overvoltage.
Various coating materials have been suggested to improve the hydrogen overvoltage characteristics of electrolytic cell cathodes in an economically viable manner. A significant number of the prior art coatings have included transition metals other than iron or steel, such as cobalt and nickel, or mixtures, alloys or intermetallic compounds of these metals with various other metals. Frequently, when nickel is employed in admixture with another metal or compound, the second metal or compound can be leached or extracted in a solution of, for example, sodium hydroxide, to provide high surface area coatings, such as Raney nickel coatings.
Copending application Ser. No. 104,235, filed Dec. 17, 1979, discloses a low hydrogen overvoltage cathode having an active surface layer comprising, as preferred embodiment thereof, a codeposit of nickel, molybdenum or an oxide thereof, and cadmium. Other transition metal-based cathode coatings are disclosed in U.S. Pat. No. 4,105,532, issued Aug. 8, 1978, and U.S. Pat. No. 4,152,240, issued May 1, 1979, which relate to cathodes comprising, respectively, alloys of nickel-molybdenum-vanadium and nickel-molybdenum using specially selected substrate and intermediate coatings of copper and/or dendritic copper. Similar coatings are also enclosed in U.S. Pat. Nos. 4,033,837 and 3,291,714.
Copending application Ser. No. 149,814, filed May 14, 1980, now U.S. Pat. No. 4,295,951 discloses a film-coated cathode useful in alkali metal halate cells. The film-forming material is a permanent coating which serves to eliminate the need for chromate additives to the cell whle providing a high operating efficiency.
U.S. Pat. No. 4,055,476, issued Oct. 24, 1977, discloses the continuous addition of nickel-based catalysts to an electrolytic diaphragm cell brine feed to prevent the formation of chlorates in the cell by decomposing sodium hypochlorite. Other reagents which are disclosed as being useful for this purpose include hydrochloric acid, sodium tetrasulfide, and various nickel and cobalt compounds. However, this patent does not recognize the utility of any of the above-mentioned materials for the prevention of cathode dissolution during periods of current interruption or cell shutdown.
Under typical commercial operating conditions of the chloralkali industry, it is not unusual for the operator of the cell to experience periods of interrupted current as frequently as once a month or more. The frequency of such shutdown periods thus poses a serious problem to the durability and maintenance of transition metal-based cathodes used in commercial cells. It will therefore be readily appreciated that a need exists for protecting such cathodes against the corrosive effects of sodium hypochlorite during shutdown periods.